Method of forming low-resistance contact electrodes in semiconductor devices

ABSTRACT

There is formed on a semiconductor substrate a lamination of a first insulating film of nondoped silicon glass or the like and, on this first insulating film, a second insulating film of boron phosphor silicate glass or the like, with a conductor layer between the two insulating films. A hole is first dry-etched in the second insulating film, leaving the substrate surface covered by the first insulating film. Then the second insulating film is heated to a reflow temperature such that the hole is thermally deformed, flaring as it extends away from the insulating film. Then a second hole is dry-etched in the first insulating film through the first recited hole in the second insulating film, with the consequent exposure of the semiconductor surface. Then a contract electrode is fabricated by filling the first and the second hole with an electroconductive material into direct contact with the substrate surface. Being covered by the first insulating film, the substrate surface is not to be contaminated with impurities during the heating of the second insulating film.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of fabricating contactelectrodes in semiconductor devices such as integrated circuits,transistors, and diodes. The method of this invention is particularlywell applicable to the fabrication of contact electrodes inpower-handling semiconductor devices in which the insulating films to bepenetrated are generally thicker than in other semiconductor devices.

[0002] As semiconductor devices have grown finer in design in recentyears, so have become less in size the contact holes that must be formedin and through their insulating films for creation of electrodes. Inaddition to that, at least as far as power-handling semiconductordevices are concerned, the insulating films in which are to be formedthe contact holes remain relatively thick in order to withstand highvoltages. The so-called aspect ratio of the contact holes, the ratio ofhole depth to diameter, have become all the more higher in this type ofsemiconductor devices, making it proportionately more difficult to fillmetal, such as aluminum, into these holes. An incomplete packing of thecontact holes with metal is of course undesirable from the standpoint ofreduction of contact resistance to a minimum.

[0003] A conventional remedy to this problem is what is known as thecontact reflow process, which starts with the creation of a hole in afilm of electrically insulating material on a semiconductor substrate,as by the more conventional method of photolithography or etching. Theinsulating film may be of either boron-phosphor-silicate glass (BSPG) orphosphor-silicate glass (PSG). Anisotropic etching is considereddesirable for creation of a hole that extends approximatelyperpendicular to the substrate surface.

[0004] The next step is the heating of the insulating film to such atemperature that it undergoes deformation, or reflow, with theconsequent flaring of the hole as it extends away from the substratesurface, or tapering thereof as it extends toward the substrate surface.Even though the insulating film may be relatively thick, and theoriginal hole correspondingly high in aspect ratio, the tapering contacthole thus formed is bound to accept metal far more easily and morethoroughly than if it were constant in diameter, as the metal isintroduced as by vacuum deposition. The result is an improvement in theso-called step coverage of the electrode.

[0005] The contact reflow process has its own shortcoming, however. Uponheating, as above, of the insulating film to a reflowing temperaturefollowing the creation of a hole therein, there occurs the so-calledoutward diffusion of such substances as phosphor and boron containedtherein. These substances, especially boron, find their way onto thesubstrate surface exposed through the contact hole thereby preventingfavorable electrical contact of the electrode with the substrate.

[0006] An obvious solution to this weakness of the contact reflowprocess might seem to create, as by thermal oxidation, a silicon oxidefilm on the substrate surface forming the bottom of the contact hole,preparatory to the heat treatment of the insulating film. This solutionwould be impractical because the noted outward diffusion of boron andthe like would occur during creation of the silicon oxide film, to suchan extent that the substrate surface would not be satisfactorily keptfrom contamination by the impurities.

SUMMARY OF THE INVENTION

[0007] The present invention represents an improvement of the contactreflow method, aiming specifically at preventing the outward diffusionof the impurities contained in the insulating film or films and reducingthe contact resistance to an absolute minimum.

[0008] The invention also seeks to attain the first recited object inthe simplest possible manner, without unnecessarily adding to the stepsof electrode fabrication.

[0009] Briefly, the present invention may be summarized as a method offabricating a contact electrode in a semiconductor device. There isfirst formed on a surface of a semiconductor substrate a lamination of afirst and a second insulating film of different materials, possibly withany required conductor layer interposed therebetween. Then a first holeis etched at least in the second insulating film which overlies thefirst insulating film, thereby leaving the substrate surface covered byat least part of the thickness dimension of the first insulating film.Then the second insulating film is heated to a reflowing temperaturesuch that the first hole in the second insulating film is so reshaped asto flare as it extends away from the first insulating film. Then asecond hole is etched in the first insulating film through the firsthole in the second insulating film, with consequent exposure of thesubstrate surface at the bottom of the second hole. Then a contactelectrode is formed by filling the first and the second hole with anelectroconductive material.

[0010] It is to be noted that two insulating films of differentmaterials are formed on the semiconductor substrate according to theinvention. A hole is first etched in the overlying second insulatingfilm, leaving the substrate surface covered by the first insulating filmduring the subsequent reflow treatment of the second insulating film.This first hole is therefore deformed into a tapering shape without theleast possibility of the substrate surface being contaminated by theimpurities dispersed from the second insulating film. Fabricatedfollowing the subsequent creation of the second hole in the firstinsulating film through the first hole, the contact electrode ispredestined to make low-resistance contact with the substrate surfaceforming the bottom of the second hole.

[0011] For successfully creating the first hole so as to leave thesubstrate surface covered by at least part of the thickness dimension ofthe first insulating film, it is recommended that materials for the twoinsulating films and an etchant for use be so chosen in relation to oneanother that the first insulating film is slower to be etched than thesecond. With the first insulating film thus left shielding the substratesurface by taking advantage of the difference between the etching ratesof the two insulating films, the contamination of the substrate surfacewill be prevented without unnecessarily complicating the process ofelectrode fabrication.

[0012] It will also be appreciated that the tapering shape of the firsthole remains intact upon creation of the second hole down to thesubstrate surface. Thus the invention saves the advantages of the priorart reflow process.

[0013] The above and other objects, features and advantages of thisinvention will become more apparent, and the invention itself will bestbe understood, from a study of the following detailed description andappended claims, with reference had to the attached drawings a preferredmode of carrying out the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIGS. 1 through 6 is a series of schematic illustrationssequentially showing the steps of fabricating a low-resistance contactelectrode on a semiconductor device by the method of this invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] There may be first prepared a semiconductor substrate 10, FIG. 1,in the form of a single crystal of silicon. Although the showing of thisfigure, as well as the showings of all the figures that follow, ishighly simplified to illustrate the invention in its simplest form, itis understood that the substrate 10 has a plurality of semiconductorregions formed as by diffusion or epitaxial growth for fabrication ofdesired semiconductor devices such as transistors and diodes.

[0016] Then, as shown in FIG. 2, a first insulating film 11 of nondopedsilicon glass (NSG) is formed on one major surface of the semiconductorsubstrate 10 as by familiar chemical vapor deposition. Then a conductinglayer 18 is formed on part of the first insulating film 11 for use inelectrical connection of the semiconductor device. Then a secondinsulating film 12 of BSPG is formed, also as by chemical vapordeposition, on the entire surface of the first insulating film 11,inclusive of that of the conducting layer 18. The first insulating film11 may be approximately 0.2-0.7 micrometer, and the second insulatingfilm 12 approximately 0.5-1.8 micrometers, in thickness. The firstinsulating film 11 is higher in melting point than the second 12.Functionally, the two insulating films 11 and 12 serve to insulate theconductor layer 18 from the conductor or semiconductor regions withinthe substrate 10 and from the conductors, not shown, to be formed on theinsulating film 12.

[0017] Then the second insulating film 12 is masked with an etchantresist 13, FIG. 3, which has defined therein an opening 13 a throughwhich a contact hole is to be formed. The second insulating film 12 issubsequently etched through the masking of the etchant resist 13 to havea hole 14 created therein just under the etchant resist opening 13 a.The etching process employed here is dry etching, with use of a gaseousetchant that can etch the second insulating film 12 so much faster thanthe first insulating film 11 that, as depicted also in FIG. 3, the firstinsulating film mostly, but perhaps not wholly, remains intact as thehole 14 is created all through the second insulating film. The knownreactive ion etching process, using an anisotropic gas etchant, isparticularly preferred for the purposes of the invention. The etchinggas in use may be that containing carbon and fluorine as in the form offluoroform or trifluoromethane, CHF₃, or tetrafluoromethane or carbontetrafluoride, CF₄.

[0018] A closer study of FIG. 3 will reveal that the first insulatingfilm 11 is shown slightly removed by the above etching process. Thisshowing is to indicate that such partial etching of the first insulatingfilm 11, along with the complete etching of the required part of thesecond insulating film 12, is allowable in practice. All that isrequired at this juncture is that the surface of the substrate 10 remaincovered by the first insulating film 11, either by part or whole of itsthickness dimension. It will also be observed from FIG. 3 that the hole14 is of practically constant diameter, extending normal to the surfaceof the substrate 10, thanks to the anisotropic etching process employed.

[0019] Then, following the removal of the etchant resist 13, the secondinsulating film 12 is heated to a temperature range of approximately900°-1000° C. Thereupon, as has been set forth in conjunction with theconventional contact reflow process, the second insulating film 12 ofBSPG will be thermally caused to reflow with the consequent reshaping ofthe constant-diameter hole 14 into a hole 14 a, FIG. 4, that flares asit extends away from the remaining first insulating film 11, or,speaking conversely, tapers as it extends toward the same. Fabricatedfrom NSG, the first insulating film 11 will be unaffected by this heattreatment of the second insulating film 12. The heat treatment will,besides reshaping the hole 14, cause the noted outward diffusion of thephosphor and boron contained in the second insulating film 12. Unlikethe prior art process, however, these diffusing impurities will notcontaminate the surface of the silicon substrate 10 through the hole 14a, the substrate surface being still left covered by at least part ofthe thickness dimension of the first insulating film 11.

[0020] Then a hole 15, FIG. 5, is created as by dry etching in the firstinsulating film 11 in register with the tapering hole 14 a in the secondinsulating film 12. The second insulating film 12 serve to mask thefirst insulating film 11. This hole 15 will be hereinafter referred toas the second hole, and the tapering hole 14 a as the first hole.Preferably, the second hole 15 may also be formed by the known reactiveion etching process with use of a gaseous etchant containing CHF₃ andCF₄, in order that the second hole may extend normal to the surface ofthe substrate 10 as shown. Formed in this manner, the second hole 15will constitute a streamlined extension of the first hole 14 a, bothholes being practically equal in diameter at the junction between thetwo insulating films 11 and 12. The reference numeral 16 in FIG. 5generally denotes the contact hole comprised of the first hole, or firsthole part, 14 a which tapers as it extends through the second insulatingfilm 12 toward the first insulating film 11, and the second hole, orsecond hole part, 15 which extends with a constant diameter through thefirst insulating film down to the surface of the substrate 10.

[0021] Next comes the step of the creation of a metal-made contactelectrode 17, FIG. 6, in the contact hole 16. Formed as by vacuumdeposition of aluminum, the electrode should overfill the contact hole16, making low-resistance contact with the surface of the substrate 10and appropriately overflowing from the contact hole. Any required numberof such contact electrodes, contacting different semiconductor regions,may be formed simultaneously in practice.

[0022] The advantages gained by the above-described exemplary method ofthis invention may be recapitulated as follows:

[0023] 1. The electrode 17 has an improved stop coverage thanks to thefunnel-shaped first part 14 a of the contact hole 16.

[0024] 2. Left covered by the first insulating film 11 during the heattreatment of the second insulating film 12 for creation of the taperingfirst hole part 14 a, the substrate surface is protected fromcontamination by the impurities contained in the second insulating film12. Consequently, the electrode 17 formed subsequently in the contacthole 16 infallibly makes low-resistance contact with the substratesurface.

[0025] 3. Only the second insulating film 12 is etched away, leaving thefirst insulating film 11 covering the substrate surface, by takingadvantage of a difference in etching rate between the two insulatingfilms 11 and 12. The objective of preventing an increase in contactresistance due to the outward diffusion of the impurities is thusrealized by the simplest possible method.

[0026] Notwithstanding the foregoing detailed disclosure it is notdesired that the present invention be limited by the exact showing ofthe drawings or the description thereof. The following is a brief listof possible modifications of the illustrated exemplary method:

[0027] 1. The second insulating film 12 only could be etched away forcreation of the hole 14 a, instead of etching part of the firstinsulating film 11, too, as in the representative method. This option isnot recommended, however. For, should the second insulating film 12partly remain at the bottom of the hole 14, this second insulating filmmight be so deformed, during the heat treatment for reshaping the hole14 into the tapering hole 14 a, as to fill the hole 14. This danger iseliminable by etching the second insulating film 12 all the way down to,and even partly into, the first insulating film 11 as through adjustmentof the etching time.

[0028] 2. An additional insulating film or films may be interposed asrequired between semiconductor substrate 10 and second insulating film12.

[0029] 3. The holes 14 and 15 could be formed by such familiar dryetching methods as sputter etching or ion beam etching, or even by wetetching.

[0030] 4. The electrode 17 could be formed by plating.

[0031] 5. The electrode 17 could be a lamination of two or moredifferent materials.

[0032] All these and other changes and adaptations of the illustratedmethod are intended in the foregoing disclosure. It is thereforeappropriate that the invention be construed broadly and in a mannerconsistent with the fair meaning or proper scope of the subjoinedclaims.

What is claimed is:
 1. A method of forming a contact electrode in asemiconductor device, which comprises: (a) providing a semiconductorsubstrate having a surface; (b) forming a first insulating film on thesurface of the semiconductor substrate; (c) forming a second insulatingfilm on the first insulating film, the first and the second insulatingfilm being different in material; (d) creating a first hole at least inthe second insulating film by etching, thereby leaving the surface ofthe semiconductor substrate covered by at least part of the thicknessdimension of the first insulating film; (e) heating the secondinsulating film to a reflowing temperature whereby the first hole in thesecond insulating film is reshaped so as to flare as it extends awayfrom the first insulating film; (f) creating a second hole in the firstinsulating film by etching the same through the first hole in the secondinsulating film, thereby exposing part of the surface of thesemiconductor substrate; and (g) forming a contact electrode by fillingthe first and the second hole with an electroconductive material.
 2. Amethod of forming a contact electrode as set forth in claim 1, whereinthe first hole is created using an etchant such that the secondinsulating film is etched at a higher rate than is the first insulatingfilm.
 3. A method of forming a contact electrode as set forth in claim1, wherein the first hole is created by anisotropic etching.
 4. A methodof forming a contact electrode as set forth in claim 1, wherein thefirst hole is created by dry etching.
 5. A method of forming a contactelectrode as set forth in claim 1, wherein the second hole is created byanisotropic etching.
 6. A method of forming a contact electrode as setforth in claim 1, wherein the second hole is created by dry etching. 7.A method of forming a contact electrode as set forth in claim 1, whereinthe first insulating film is of nondoped silicon glass, and the secondinsulating film of boron phosphor silicate glass.
 8. A method of forminga contact electrode as set forth in claim 7, wherein the first hole iscreated using a gaseous etchant containing carbon and fluorine.
 9. Amethod of forming a contact electrode as set forth in claim 7, whereinthe second hole is created using a gaseous etchant containing carbon andfluorine.
 10. A method of forming a contact electrode as set forth inclaim 1, wherein the first insulating film is from abut 0.2 to about 0.7micrometer, and the second insulating film from about 0.5 to 1.5micrometers, in thickness.
 11. A method of forming a contact electrodeas set forth in claim 1, wherein the first insulating film is higher inmelting point than the second insulating film.